Batteries are frequently used as sources of motive and control power in certain applications where a temporary loss of power would not be acceptable. These applications are primarily industrial, such as boiler controls, computer power supplies and even sensitive control systems in nuclear power plants. Similarly, there are commercial applications where a in backup power supply must be provided, such as in hospitals and backup lighting systems which function when normal AC power is lost. In some cases, batteries are only used as a backup source of power when normal AC power is lost. In other cases, batteries are used as the normal source of power. Consequently, in either case, batteries must be charged periodically such that they are able to supply power as required.
Typically battery chargers use a source of AC power which is rectified to DC to charge the batteries. There are several types of battery chargers. One type is a series pass battery charger, wherein a series pass element is connected between the input voltage source and the DC output terminals. This type of regulator is also called a dissipative type because of the magnitude of power dissipation by the series pass element during certain modes of operation. When the battery is deeply discharged, the power dissipated by the series pass element is at a maximum and is equal to the difference between the input and output voltages multiplied by the maximum charging current. When the battery is deeply discharged, battery voltage is insignificant relative to the input voltage supply source. Therefore, the power dissipated by the series pass element is essentially the input supply voltage multiplied by the charging current. If this condition occurs coincident with a higher than normal input voltage, the power dissipation will be even greater. During these conditions of high power dissipation by the series pass element, overall efficiency of the charger is very poor. Because of the low efficiency of the series pass regulators, other regulator types have become more popular.
Another type of battery charger, the switching type, is used when charger efficiency is of major importance. Switching type battery chargers are also known as non-dissipative battery chargers. This type of charger stores excess power in a filter circuit, delivering power to the battery in measured intervals when a switching element conducts. Power is stored in the filter and delivered to the battery as required. With this approach, little power is dissipated and hence the efficiency is much higher. Examples of switching type regulators can be found in U.S. Pat. Nos. 3,304,489 to Brolen, 4,061,956 to Brown and 4,359,679 to Regan. Although, switching type chargers are more efficient, they are not without drawbacks. One drawback found with this type of charger is that it generates electromagnetic interference. Consequently it requires filtering circuits, whereas series pass regulators generate little or no electromagnetic interference.
Another type of battery charger is the Silicon Controlled Rectifier (SCR) battery charger. In this type of charger, an SCR is used to control the input voltage applied to the filter and the battery as in the switching type. Advantages and disadvantages of this type of battery charger are similar to that of a switching type battery charger. Examples of SCR type regulators can be found in U.S. Pat. Nos. 3,420,440 to Bixby and 3,543,127 to Fry.
Both the switching battery charger and the SCR controlled battery charger have more components than the series pass charger. As a general rule, the more components used in a device, the less reliable it becomes. Also, increasing the number of components adds to cost and complicates troubleshooting because the control circuits are generally more complex. Therefore, even though switching and SCR controlled chargers may be desirable from an efficiency standpoint, they are not as desirable as the series pass charger from a reliability standpoint. Since batteries and battery chargers are often used in situations where a loss of AC power cannot be tolerated, reliability is often a more important factor than efficiency. Therefore, any improvement in the efficiency of series pass battery chargers would be welcomed by the industry. More importantly the lack of an acceptable solution to improving the efficiency of series pass battery chargers demonstrates that there is a long felt need by the industry which has heretofore eluded those skilled in the art.